The present invention is related in general to the field of semiconductor devices and processes, and more specifically to the materials and fabrication of leadframes for integrated circuit devices.
Leadframes for semiconductor devices provide a stable support pad for firmly positioning the semiconductor chip, usually an integrated circuit (IC) chip within a package. Since the leadframe, including the pad, is made of electrically conductive material, the pad may be biased, when needed, to any electrical potential required by the network involving the semiconductor device, especially the ground potential.
In addition, the leadframe offers a plurality of conductive segments to bring various electrical conductors into close proximity of the chip. The remaining gap between the inner tip of the segments and the contact pads on the IC surface are typically bridged by thin metallic wires individually bonded to the IC contact pads and the leadframe segments.
Also, the ends of the lead segment remote from the IC chip (xe2x80x9couterxe2x80x9d tips) need to be electrically and mechanically connected to external circuitry, for instance to assembly printed circuit boards. In the overwhelming majority of electronic applications, this attachment is performed by soldering, conventionally with lead-tin (Pb/Sn) eutectic solder at a reflow temperature in the 210 to 220xc2x0 C. range.
Finally, the leadframe provides the framework for encapsulating the sensitive chip and fragile connecting wires. Encapsulation using plastic materials, rather than metal cans or ceramic, has been the preferred method because of low cost. The transfer molding process for epoxy-based thermoset compounds at 175xc2x0 C. has been practiced for many years. The temperature of 175xc2x0 C. for molding and mold curing (polymerization) is compatible with the temperature of 210 to 220xc2x0 C. for eutectic solder reflow.
Reliability tests in moist environments require that the molding compound have good adhesion to the leadframe and the device parts it encapsulates. Two major contributors to good adhesion are the chemical affinity of the molding compound to the metal of the leadframe and the surface roughness of the leadframe. The recent general trend to avoid Pb in the electronics industry and use Pb-free solders, pushes the reflow temperature range into the neighborhood of about 260xc2x0 C. This higher reflow temperature range makes it more difficult to maintain the mold compound adhesion to the leadframes required to avoid device delamination during reliability testing at moisture levels. Known leadframes do not offer metallization for good adhesion combined with low cost, easy manufacturability, and avoidance of whiskers.
It has been common practice to manufacture single piece leadframes from thin (about 120 to 250 xcexcm) sheets of metal. For reasons of easy manufacturing, the commonly selected starting metals are copper, copper alloys, and iron-nickel alloys (for instance the so-called xe2x80x9cAlloy 42xe2x80x9d). The desired shape of the leadframe is etched or stamped from the original sheet. In this manner, an individual segment of the leadframe takes the form of a thin metallic strip with its particular geometric shape determined by the design. For most purposes, the length of a typical segment is considerably longer than its width.
Nickel plating of the leadframe starting metal has been shown to be desirable because nickel reduces the propensity for tin dendrite/whisker growth in devices with tin-plated leads, a generally feared failure phenomenon. Nickel, however, has poor adhesion to most molding compounds. Therefore, it is typically coated with a thin layer of palladium or gold. The pressure of cost reduction, though, in semiconductor manufacturing requires that noble metals be used cautiously.
A need has therefore arisen for a low cost, reliable leadframe combining adhesion to molding compounds, bondability for connecting wires, pro-plating of tin, and avoiding the risk of tin whisker growth. The leadframe and its method of fabrication should be low cost and flexible enough to be applied for different semiconductor product families and a wide spectrum of design and assembly variations, and should achieve improvements toward the goals of improved process yields and device reliability. Preferably, these innovations should be accomplished using the installed equipment base so that no investment in new manufacturing machines is needed.
One embodiment of the invention is a leadframe for use in the assembly of integrated circuit (IC) chips, which has first and second surfaces. The leadframe includes a base metal structure with an adherent layer comprising nickel having a rough, non-reflecting surface covering the base metal. This rough nickel enhances adhesion to molding compounds. Adhering to the rough nickel layer is a layer comprising smooth, reflective nickel, selectively covering the first surface of the leadframe in areas intended for attachment of bonding wires and the IC chip. This smooth nickel supports vision systems in wire bonders and the pick-and-place machines.
In another embodiment of the invention, a further adherent layer of metal is deposited in selected areas of the first leadframe surface to provide attachment of bonding wires as well as attachment to external parts.
In another embodiment of the invention, an adherent metal layer suitable for wire bond attachment is deposited in the region for wire bonding. Then, the transfer molding process is performed. Finally the external leads of the molded device are plated with a metal or alloy suitable for attachment to external parts.
In another embodiment of the invention, a first adherent metal layer is deposited on the leadframe in selected areas for wire bond attachment, and a second adherent metal layer is deposited to provide attachment to external parts.
Embodiments of the present invention are related to high density ICs, especially those having high numbers of inputs/outputs, or contact pads, and also to devices in packages requiring surface mount in printed circuit board assembly. These ICs can be found in many semiconductor device families such as standard linear and logic products, digital signal processors, microprocessors, wireless devices, digital and analog devices, and both large and small area chip categories. The embodiments provide a significant cost reduction, provide improved adhesion to molding compounds and manufacturability in wire bonding, suppress tin whisker formation, and enhance environmental protection and assembly flexibility of semiconductor packages, especially the plastic molded packages, compared to the conventional copper-based solder-plated leadframes.
It is a technical advantage of one or more embodiments of the invention that the embodiments can reach the goals of the invention with a low-cost manufacturing method without the cost of equipment changes and new capital investment, by using the installed fabrication equipment base.
Another advantage which may flow from one or more embodiments of the invention is to produce leadframes so that established wire bonding processes can continue unchanged, and that established board attachment process can continue unchanged. As an example, in one embodiment the leadframes prepared according to the invention can be successfully used in surface mount technologies based on bending the package lead segments. Embodiments of the invention generally apply to semiconductor package types such as PDIPs, SOICs, QFPs, SSOPs, TQFPs, TSSOPs, TVSOPs, and Ball Grid Array devices employing leadframes.
The technical advances represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.